Radio receiver



Oct 18, 1938, w CARLSQN 2,133,808

' RADIO RECEIVER Filed Sept. 27, 1933 2 SheetsSheet l m i 2 J INVE'NTOR WendqllL Cdrlqon act. 18, 1938. w CARLSQN 2,133,808

RADIO RECEIVER I Filed Sept. 27, 1933 2 Sheets-Sheet 2 NORMAL sELEcT/v/TY 0F c/Rcu/Ts A flND A COMBINED CIRCUIT H For? SIDE SELECT! v1 7' v OF BAND RECEPTION SELECT/wry OFCIRCUIT A UNCHANGED FOR 6105 BAND RECEPTION COMB/NED SELECT/V17) 2 OF CIRCUITS A AND/7' 155 E F'Ol? SIDE .BHND RECEPTION FREQUENCY SIDE BEND SIDE BAND CARRIER NORMAL SELECT/WT) R 14.9 OF B AND C CIRCUITS SELEC'T/V/TY 0F 9.,q.v.c. 1

CRCUIT SELECTIV/TY OF C CIRCUIT FOR 15 SIDE BHND HEEPTION SELECTIV/TY 0F Bc/Rcu/r FOR SIDE BAND 3 RECEPTION g =1: (J

FREQUENCY v T -v SIDE BEND SIDED/1ND CARRIER i t {D FREQUENCY INVENTOR WendelLL. Carlson BY g! Patented Oct. 18, 1938 RADIO RECEIVER Wendell L. Carlson, Haddonfield, N. J.,'assignor' to Radio Corporation of America, a corporation of Delaware Application September 27, 1933, Serial No. 691,120

16 Claims My invention relates to radio receivers and particularly to a receiver designed to be operated either-as a conventional highly selective receiver or as a high fidelity receiver designed to suppress part of one side band of the incoming signal.

The usual broadcast transmitter transmits .a signal which consists of a carrier wave and the upper and lower side bands. .Theconventional receiver is tuned to the carrier wave frequency so that equal amounts of the two side bands are received. With a crowded broadcast band such a receiver generally must be so sharply tuned that the high frequency components of the side bands are cut off, whereby the quality of the reproduced voice or music from a received modulated carrier wave is impaired.

The cutting off of the side bands may be avoided by merelybroadening the tuning of the receiver circuits, but this is objectionable for two reasons. In the first place, it greatly lowers the gain of the receiver, and in the second place, it permits interference from stations on adjacent channels.

It is, accordingly, an object of my invention to provide animproved receiver which may be operated selectively either as a conventional receiver or as a high fidelity receiver.

It is a further object of my invention to provide a high fidelity receiver that shall have comparatively high gain and improved selectivity.

It is a still further object of my invention to provide an improved receiver for receiving mainly the carrier and one side band of a radio signal wave.

In practicing my invention, and in a preferred embodiment thereof, I provide a superheterodyne receiver with a control circuit which is sharply tuned to the intermediate frequency for the purpose of making the receiver inoperative until it is tuned exactly to the carrier frequency of the desired station. Operated in this manner, the receiver is sharply selective. I further provide switching means for broadening the tuning of certain circuits on one side of the carrier frequency only, whereby the receiver may be operated as a less sharply selective, high fidelity receiver. The tuning is broadened. sufiiciently to permit the passage of one complete side band. The other side band, except the lower frequency end thereof, is appreciably attenuated or suppressed.

It is evident that the receiver, when operating as a single side band, high fidelity receiver must always be tuned to a single side band of an incoming signal before the above-mentionedcontrol circuit'j permits the-receiver to operate. In other words, the receiver must bev tuned to one side of the middle of the main channel.

Other features and advantages of my invention will appear from the following description 5 taken inconnection with the accompanying drawings, in which" I Figure 1 is a circuit diagram of a receiver-constructed' in accordance with oneembodiment of my invention;v Figs. 2 and 3 are curves showing the selectivity characteristics of certain tuned circuits of the receiver shown in Fig. 1; and I 1 Fig. 4 shows certain signal output characteristics of the receiver shown in Fig. 1. 1: a

Referring to the embodiment of my invention shown in Fig. 1,.itcomprises a superheterodyne receiver. which includes a radio frequency am:- plifier tube I, a first detector tube 3, an intermediate frequency amplifier tube 5,v and a second detector .tube 1. These amplifier and detector tubes may be any suitable type of space discharge device. In. the example illustrated, they are conventional screen grid and three element tubes. 5

The amplifier tube l is provided with an input circuit which includes a radio frequency transfo-rmer' 9 having a primary winding H and a secondary winding I3, the primary winding ll being connected toan antenna l5 and to ground.

In order .to make the winding 13 tunable at radio frequencies, it is shunted by a variable tuning condenser H, a blocking condenser l9, and a fixed tuning j.condenser 2|, all connected in series. This tunable circuit will be referred to as the'selecting or filter circuit A. The fixed tuning. condenser 2| is provided with a short circuiting switch 23 for changing the tuning of the selecting circuit A when the receiver is to be adjusted for single side band reception. The 40 control grid of the amplifier tube l is provided with a constant negative bias by means of a grid biasing resistor 25 connected in the cathode-lead and shunted by a radio frequency bypass condenser 21. It is also provided with a variable biasing or controlling potential as hereinafter described.

The'amplifier tube l is provided with an output circuit which is coupled to the input circuit of the detector tube 3 through a transformer 29. The secondary 3| of the transformer 29 is shunted by a variable tuning. condenser '33 and a blocking condenser 35 connected in series to form a second radio frequency filter circuit A. The condensers l1 and 33-are units of, a gang condenser which are operated by a common control as indicated by the dotted line 31.

When the receiver is operated as a conventional, sharply selective receiver, the condenser 2| is short-circuited and the tuned circuits A and A are tuned to the same frequency. When operated as a high fidelity receiver, the switch 23 is open and the circuit A is tuned to a slightly higher frequency than the circuit A.

The superheterodyne oscillator 39, which is coupled as indicated, to the first detector 3 for producing the intermediate frequency signal, may be of any suitable type. It includes a tuning condenser 4| which is operated as indicated, by the above-mentioned common control for setting the oscillator at a frequency above that to which the radio frequency circuits are tuned by an amount equal to the intermediate frequency The detector tube 3 is provided with an output circuit which is coupled to the input circuit of the intermediate frequency amplifier tube 5 through a transformer 43.

The primary winding 45 and the secondary winding 41 of the transformer 43 are tuned by means of condensers 49 and 5|, respectively, to form a band pass filter B of the type commonly employed in superheterodyne receivers. The tuning condenser 49 is connected in series with a second tuning condenser 53 which is provided with a short circuiting switch 55. When the switch 55 is closed, the band pass range of the filter B is madewider and shifted to a slightly lower frequency.

The control grid 56 and the cathode 58 of the intermediate frequency amplifier tube 5 are connected to the tuned secondary circuit of the filter B. The connection of the cathode 58 to this circuit is made through automatic volume control circuits which will be described hereinafter.

The control grids of both the first detector tube 3 and the intermediate frequency tube 5 are supplied with the proper constant bias by means of resistors 51 and 59 in the cathode leads, these resistors being shunted by radio frequency bypass condensers BI and 83, respectively.

The output of the amplifier tube 5 is fed to the input circuit of the second detector tube 1 through a radio frequency transformer 65 having a primary winding 61 and a secondary winding 69. Tuning condensers: H and 13 are connected across the windings 61 and 69, respectively, to form a band pass filter 0 having a predetermined selectivity characteristic.

In order to broaden the tuning of the filter C for high fidelity reception, a coupling coil I5 is connected in series with the secondary winding 69 and coupled as indicated, to the primary winding 61. A switch 11 is provided for connecting the tuning condenser 13 across or in parallel with either the secondary winding 89 itself, or the secondary winding 89 and the winding 15 in series. By throwing the switch IT to the position shown to connect condenser 13 across both windings, the tuning of the filter C is broadened to avoid the cutting off of the carrier wave side bands. This broadening of the tuningis caused by the increase in coupling between the primary and secondary circuits, as explained in my Patent No. 1,871,405. The amount of inductance in coil 15 and the degree of coupling to primary 61 are adjusted so as to obtain the desired frequency range with the same increase of frequency range on each side of resonance compared to the sharp resonant circuit.

The audio frequency output of the second detector I is supplied to a suitable audio frequency amplifier or translating device (not shown) through an audio frequency transformer 19.

The above described superheterodyne receiver is provided with an automatic volume control or A. V. C. circuit which includes an intermediate gequency amplifier tube 8| and a rectifier tube In the particular embodiment shown, the amplifier tube 8| is of the screen grid type having a control grid which is maintained at the desired negative potential through a biasing resistor in the cathode lead.

. The input circuit of the amplifier 8| is coupled to the output circuit of the band pass filter B by means of a coupling condenser 81 and grid impedance 88.

The amplifier tube 8| is provided with an output circuit which is coupled to the input circuit of the rectifier tube 83 through a three windingtransformer 89. The primary winding 9| of the transformer 89 is tuned to the intermediate frequency'by means of a condenser 93. The secondary winding is in the rectifier circuit andlis closely coupled to the primary winding 9| to broaden the frequency range of the transformer.

The input circuit of the rectifier tube 83 includes a resistor 91 and a source of biasing potential such as a biasing battery 99, connected in series with the secondary winding 95 and the resistor 91.: 'When intermediate frequency signal energy isfed to rectifier 83, a voltage drop having a value depending upon the strength of an incoming signal will appear across resistor 91. The voltage drop across the entire resistor 91 is applied to the control grids of amplifier tube and detector tube 3 through a conductor NH and filter resistors Hi3 and I95. A filter condenser I01 connected between the end of the resistor 91 and ground prevents audio frequency signals from appearing in the output of the volume con- The voltage drop across aportion of the resistor 91 is applied to the control'grid 56 of the intermediate frequency amplifier tube 5 through a conductor I09. This portion of the resistor is shunted by another filter condenser Ill.

The biasing battery 99 prevents the A. V. C. circuit from becoming effective to reduce the gain of the receiver until after the strength of the incoming signal exceeds a predetermined value, since the anode of rectifier 83 is maintained negative until the signal voltage across the secondary 95 exceeds said value.

In accordance with my invention, the receiver is also provided with a highly selective circuit tuned to the intermediate frequency for maintaining the receiver in an inoperative condition except at the times that the said highly selective circuit is passing energy; This circuit, which may be referred to as the quiet automatic volume control or Q. A. V. C.,circuit, includes an intermediate frequency amplifier tube ||3, a rectifier tube I I5 and a control tube H1.

The intermediate frequency amplifier tube ||3, which is of thescreen grid type in the example illustrated, has its input electrodes coupled to the output circuit of the preceding amplifier tube -8| by means of the other secondary winding H9 of the intermediate frequency transformer 89, the secondary winding 9 being loosely coupled and sharply tuned to the intermediate frequency by means of a condenser |2|. The intermediate frequency amplifier H3 is provided with an out-- put circuit coupled to the inputcircuit of the rectifier tube I I 5 through an intermediate frequency transformer I 23. The transformer I 23issharply tuned to the intermediate frequency by means of condensers I25 and I21.

The control tube II! is of the three-electrode type having a cathode I29, a control grid I3I, and an anode I33. The cathode I29 is connected through a conductor I to the cathode end of the biasing resistor 59 of the intermediate frequency amplifier 5.

The anode I33 is supplied with positive potential from any suitable source such as a battery I31 connected between the anode I33 and ground. It will be seen that so long as current fiows through the control tube Hi, there is flow of current through the biasing resistor 59 of amplifier tube 5 in such a direction as to increase the negative bias of the control grid 56. The Q. A. V. C. circuit is so adjusted that normally this flow of current is suificient to make the control grid 56 so negative that the amplifier tube 5 is made ineffective and the receiver is substantially inoperative to receive signals.

The control grid I3I of the control tube II! is connected to the input circuit of the rectifier tube II 5 at a point between the secondary winding I39 of the transformer I23 and a resistor I4I connected in series therewith and shunted by a filter condenser I45. The cathode I29 of the control tube II! is connected-to the cathode I43 of the rectifier tube I I5 whereby any potential drop appearing across the resistor MI is applied to the grid I3I of the control tube Ill.

So long as no signal reaches the rectifier tube II5, the grid of the control tube II! is in the same potential as its cathode I29, and the current flow through the control tube I I1 maintains the intermediate frequency amplifier 5 in an inoperative condition. As soon as intermediate frequency energy is impressed upon the rectifier II5, however, the grid of the control tube II! is made negative with respect to its cathode I29,

due to rectified current flowing in resistor I4I,

whereby the current flow through the control tube is either decreased or stopped entirely, thereby reducing or removing the high negative potential from the control grid 59 of the intermediate frequency amplifier 5 and rendering the receiver operative to receive signals.

The specific A. V. C. and Q. A. V. C. circuits described in this application are disclosed and claimed in application Serial No. 648,422, filed December-22, 1932, as the joint invention of W. L. Carlson and L. R. Kirkwood, and assigned to the Radio Corporation of America.

The operation of the receiver will be more clearly understood by referring to the curves shown in Figs. 2 and 3. In Fig. 2, the curves show the selectivity of the radio frequency circuits A and A for the conditions both of high selectivity and high fidelity.

For the condition of high selectivity, the switch 23 is closed and the circuits A and A are tuned to the same signal frequency, the selectivity of the two circuits combined being shown by the dotted line curve I41. For this condition of operation, the switch 55 of filter B is-open and switch TI of filter C is in its upper position to give filters B and C a selectivity characteristic shown by the dotted line curve I49 in Fig. 3.

The selectivity of the Q. A. VpC. circuit is shown by the curve I5I in Fig. 3. It will be noted that for the condition of high receiver selectivity,

the Q. A. V. C. circuitis tuned to the middle of the pass range of the filters B and C'.

With the switches thrown to the positions shown in Fig. 1 for high fidelity reception, the resonance curve of circuit A is shifted to ahigher frequency as shown by the curve I53 in Fig. 2.,

the resonance curve I55 of circuit A remaining unchanged. The combined selectivity of circuits A and A is represented by the curve I57; I

Also, as shown by the curve I59 in Fig. 3, the

pass range of filter B is widened and shifted to a lower frequency range which includes all the lower side band of the intermediate frequency signal. The pass range of filter C is increased on both sides of the intermediate frequency, that is the tuning of the filter is merely broadened as indicated by the curve I6I.

' An inspection of Figs. 2 and 3 shows that at the normal or highly selective setting of the receiver, the high frequency components of the side bands are cut off both by the circuits A and A and by the band pass filters B and C. It'will be seen, however, that at the high fidelity setting of the receiver, that is, with the switches 23, 55,

and T! in the positions shownin Fig. 1, theentire upper side band of the radio frequency carrier is transmitted together with the lower frequency end of the lower side band. It will be noted that the upper side band of the radio frequency carrier wave corresponds to the lower side band of the intermediate frequency carrier wave, as indicated in Fig. 3. This side band inversion follows from the fact that the superhetero-dyne oscillator frequency is higher than the signal frequency.

For the condition of high fidelity reception, the Q. A. V. C. circuit is tuned to a frequency within but at one end of the pass range of the filter B as will be apparent by comparing curves I5I and I59 in Fig. 3. This new relation between the filter circuits, it will be understood, is caused merely by operating a'switch control to shift the tuning of the circuits A, B, and C. Because of this relation, the receiver, when setfor'high fidelity reception, must always be tuned to one side band of a carrier wave before a signal appears in the receiver output'circuit.

The curves in Fig. 4 show the audio frequency gain of the receiver plotted against frequency for the two conditions of operation. The curve indicated at I63 is for the condition of high selectivity, while the curve indicated at I65 is for the condition of high fidelity reception.

The most important difference between thetwo curves'is that the gain for high fidelity reception holds up at the high frequency end after the gain for normal reception has dropped to a low value. It will be noted that the curve I55 shows the gain to be higher for low frequency signals than for the high frequency signals. This difference in gain is caused by the fact that the low frequency signal components of .both side bands are passed by the filter circuits.

Although the receiver has been described as adjusted for reception of the upper side band of the radio frequency. carrier wave, it could equally well be adjusted for reception of the lower side band only. Furthermore, various otherswitching arrangements for controlling the selector circuits may be provided. For example, the tuning of circuit A may be shifted in place of the tuning of circuit A; the tuning of the secondary circuit of filter B instead of the primary circuit may be changed; and the band pass characteristic of filter C may be shifted the same as for filter B although the circuit illustrated is advantageous since the gain is not changed when the tuning is changed.

It will be apparent that, instead of changing the capacity in circuits A and B, the inductance may be changed by proper switching. In the case of circuit A, however, the capacity change is preferred since it broadens the tuning the most at the lower carrier frequencies. At the higher frequencies, as at the upper end of the broadcast band, the tuning is normally rather broad and need not be broadened appreciably by the switching.

My invention is of particular importance where the signals to be amplified have such a high frequency that it is difficult to obtain the desired amplification. In television reception, for example, with the circuits tuned broadly enough to receive both side bands with high fidelity, the maximum gain per stage is about 4. By employing my invention for high fidelity reception, the gain per stage may be madeabout 6.

My invention provides improved selectivity since on one side of the received carrier there will be a wide spacing between the cut-off frequency of a filter and the adjacent transmission channel. This improvement in selectivity will be apparent by comparing curves I59 and l6l in Fig. 3 since, if the tuning of the circuits were merely broadened for high fidelity reception, the selectivity curve for the receiver would be similar to the curve l6! for filter C.

A furtheradvantage of my invention resides in the simplicity of the switching arrangement, only three switches being required in the receiver shown in Fig. 1. If the tuning were broadened for receiving both side bands, it would be necessary to employ more switches.

Various other modifications may be made in my invention without departing from the spirit and scope thereof, and I desire,therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim as my invention:

1. In a superheterodyne receiver, a first detector and a tunable oscillator coupled thereto for converting an incoming radio frequency signal to an intermediate frequency signal having at least one side band, a second detector, a bandpass filter for passing signals within a band. of frequencies approximately the width of said side band, said band-pass filter being connected between said two detectors, and means including a sharply resonant filter tuned to a frequency near one end of said band of frequencies for controlling the operativeness of said receiver.

2. In a superheterodyne receiver, a first detector and a tunable oscillator coupled thereto for converting an incoming radio frequency signal to an intermediate frequency signal having at least one side band, a second detector, a bandpass filter for passing a band of frequencies approximately the width of said side band, said band-pass filter being connected between said two detectors, a sharply resonant filter coupled to said receiver at a point between said two detectors, said sharply resonant filter being tuned to a frequency near one end of said band of frequencies, means for making said receiver normally inoperative, and means for making said receiver operative in response to the passage of energy through said sharply resonant filter.

3. In a superheterodyne receiver, a first detector and a tunable oscillator coupled thereto for converting an incoming radio frequency signal to an intermediate frequency signal having at least one side band, a second detector, a bandpass filter for passing a band .of frequencies approximately the width of said side band, said band-pass filter being connected between said two detectors, a sharply resonant filter coupled to said receiver at a point between said two detectors, said sharply resonant filter being tuned to said intermediate frequency, means for making said receiver normally inoperative, and means for making said receiver operative in response to the passage of energy through said sharply resonant filter.

4. In a radio receiver, means for selecting a high frequency modulated carrier wave including a plurality of tunable signal selecting circuits at least one of which is adjustable to respond to one side band of the modulated carrier wave, an intermediate frequency amplifier comprising a plurality of tuned'signal selecting circuits at least one of which is adjustable to respond to the intermediate frequency carrier wave and side band corresponding to said selected high frequency carrier wave and side band, an automatic volume control signal rectifier device coupled to said intermediate frequency amplifier to receive the intermediate frequency carrier wave, and means for rendering said receiver operative to receive signals including a signal rectifier device and intermediate frequency coupling means therefor comprising at least one tuned coupling circuit sharply responsive to the intermediate frequency carrier wave alone.

5. In a radio receiver, means for selecting an incoming signal comprising a modulated carrier having at least one side band, means for converting said incoming signal to a signal comprising a predetermined intermediate frequency carrier having at least one side band, an intermediate frequency amplifier which includes a tuned coupling circuit, said tuned circuit being adjustable to select only said one intermediate frequency side band, and means for making said receiver operative only in response to so adjusting said converting means that said one side band falls within the pass range of said tuned circuit, an automatic volume control system having a tuned signal input circuit responsive to a frequency within the pass range of said tuned circuit, and a quiet automatic volume control system having a tuned signal input circuit more sharply responsive than said automatic volume control system to a frequency also within the pass range of said tuned circuit.

6. In a superheterodyne receiver, a first detector tube and a tunable oscillator coupled thereto for converting a modulated incoming radio frequency signal to an intermediate frequency signal having at least one side band, a second detector tube, a plurality of amplifier tubes, all of said tubes being connected in cascade, a bandpass filter for passing a band of frequencies approximately the width of said side band, said intermediate frequency being at one end of said band, said band-pass filter being connected between said two detectors, and means including a sharply resonant filter tuned to said intermediate frequency for increasing the gain of at least one of said tubes in response to passage of energy through said sharply resonant filter.

'7. In a radio receiver, means for selecting an incoming signal comprising a modulated carrier having at least one side band, means for selecting said one side band of the incoming signal, means for converting said incoming signal and side band to a signal comprising a predetermined intermediate frequency carrier having a corresponding side band, a filter for selecting said one intermediate frequency side band and carrier, and means for making said receiver operative only in response to so adjusting said converting means that said one side band falls within the pass range of said filter.

-8. In a superheterodyne receiver, means for selecting an incoming radio-frequency signal including a carrier and at least one side band, means for converting said signal to an intermediate frequency signal including at least one side band, a filter for selecting said one side band, and means including a sharply resonant filter for making said receiver operative only when erergy at said intermediate frequency is impressed upon said resonant filter, said sharply resonant filter being tuned to a frequency lying within and at one end of the frequency range of said first filter.

9. In a radio receiver, means for converting an incoming signal consisting of a modulated carrier wave having side bands to a signal consisting of a carrier wave having a predetermined intermediate frequency and having side bands, means for selecting one of said intermediate frequency side bands, said selecting means including a band-pass filter having a pass range approximately the Width of said one side band, a sharply tuned filter tuned to a frequency within and at one end of said pass range, means for impressing at least a portion of said converted signal upon said sharply tuned filter, and means for making said receiver operative only when said sharply tuned filter passes energy.

10. In a superheterodyne receiver, a first detector tube and a tunable oscillator coupled thereto for converting a modulated incoming radio frequency signal to an intermediate frequency signal having two side bands, a second detector tube, a filter connected between said detector tubes and tuned broadly to said intermediate frequency whereby a portion of said side bands are transmitted, means including a sharply resonant filter tuned to said intermediate frequency for increasing the response of said receiver in response to the passage of energy through said sharply resonant filter, and means for so changing the tuning of said first filter that it may transmit substantially all of one of said side bands and only a small portion of the other of said side bands.

11. In a superheterodyne receiver, a first detector tube and a tunable oscillator coupled thereto for converting incoming radio frequency signals to intermediate frequency signals, a second detector tube, a filter connected between said detector tubes for passing a band of frequencies, said intermediate frequency being in the middle of said band, means including a sharply resonant filter tuned to said intermediate frequency for increasing the response of said receiver in response to the passage of energy through said sharply resonant filter, and means for broadening the tuning of said first filter on one side only of said intermediate frequency when desired.

12. A radio receiver comprising means for selecting a signal having a carrier frequency and at least one side band, a filter for selecting said one side band only, and means including a sharply resonant filter for making said receiver operative only when said one side band falls within the pass range of said first filter.

13. A radio receiver comprising means for selecting a signal having a carrier frequency and at least one side band, a filter for selecting said one side band only, and means including a sharply resonant filter for making said receiver operative only when said sharply resonant filter passes energy, said sharply resonant filter being tuned to a frequency lying within and at one end of the frequency range of said first filter.

14. In a radio receiver of the superheterodyne type, a tunable radio frequency circuit for selecting an incoming signal, an intermediate frequency selecting circuit, and means for simultaneously shifting the tuning of said radio fre-' quency circuit to select one side band only of a desired signal and the tuning of said intermediate frequency circuit to select the corresponding intermediate frequency side band,

15. In a radio receiver, means for selecting an incoming signal comprising a modulated carrier having at least one side band, means for converting said incoming signal to a signal comprising a predetermined intermediate frequency carrier having at least one side band, an intermediate frequency amplifier whichincludes a tuned coupling circuit, said tuned circuit being adjustable to select only said one intermediate frequency side band, and means for making said receiver operative only in response to so adjusting said converting means that said one side band falls within the pass range of said tuned circuit.

16. In a wireless or like receiver for modulated carrier-wave transmission, signal-selective means comprising the combination of a resonant device of low damping tuned to the frequency of the carrier wave of the signal, a comparatively broadly-tuned resonant device for receiving at least one side-band of the signals at substantially uniform amplitude, means for applying the signals to both of said resonant devices, means for controlling the total output of the receiver according to the strength of the signals, and means 

